The present invention relates to paper processing and particularly to the delivery and stacking of sheets of paper into a chute.
The rate of production of printing machines such as lithographic presses and the like is limited by the speed with which the paper ejected from the machine can be received and stacked in chute or tray. The typical delivery system from an offset printing machine includes a chute open at the top and at one end and usually inclined at an angle such that sheets exiting from the machine are also directed downward at substantially the same angle to fall into the bottom of the chute or tray. The speed with which the chute can accept sheets of paper is then limited substantially by the rate at which the sheets can fall by gravity to the bottom of the chute. This rate is typically between about 6,000 and 7,500 sheets per hour for present machines. However, such machines are typically operated at about 5,000 per hour.
The sheets of paper enter the tray at a height that would be approximately the maximum limit of a stack in the tray when full. The sheet then falls to the bottom of the tray with the next sheet entering the chute at the same level and falling on top of the previous sheet and so on until the chute is full. Thus, the feeding into the chute is substantially limited to the gravity feed rate as the sheet falls downward from the machine. Of course, the sheet moves outward away from the machine under the inertia resulting from the speed with which it exited from the machine.
Other disadvantages of the systems are that the sheets tend to fall and slide across the present printed surface of the previous sheet. When the sheets are freshly printed, this can smear ink, especially on slick surfaced paper. Freshly printed sheets are also usually statically charged and tend to cling to one another.
Still other disadvantages of the prior art are that thin sheets of paper, such as onion skin and the like, tend to roll and tumble into the chute. This results in an immediate jam-up of paper in the chute, with the result that the machine must be shut down until the chute is cleared. Sheets which have been laminated by a coating of moisture also have a tendancy to bow or curl and roll into the chute jamming it. A chain delivery system which employs grippers on a chain which grip and pull each sheet out to a position above the tray and then releases it has been developed for this problem. These, however, have not been completely satisfactory.
It is frequently necessary to build up the bottom of the chute by a block or false bottom so that the sheets land and come to rest before they have a chance to curl and tumble, thus reducing the capacity of the chute so that it must be cleared more often.
Another disadvantage is that the tray must be set to a width greater than that of the paper, and paper will frequently fail to move completely to the end of the tray. Thus, the stack must then be removed and jolted or shook to jolt the edges of the sheets together for packaging.
Accordingly, it is desirable that some apparatus be available which enhances the speed and quality of the stacking of sheets of paper from a printing or other processing machine.